UEs are required, increasingly, to have a smaller physical size, smaller morphological configuration etc. which results in limited space to implement the antennas. Further, the UE is required to communicate over several different frequency bands. The supported bands can be combination of high frequency bands at ˜2 GHz and low frequency bands at 18 800 MHz. For UEs which support MIMO signals and MIMO capabilities to the network are also required to provide this signaled capability for all the frequency bands. High spatial multiplexing or diversity MIMO gains are achieved when the antennas are sufficiently separated to guarantee a certain degree of decorrelation between the channel paths. For the low frequency bands the distance between the antennas needed in order to achieve sufficient decorrelation can be too high considering the limited space available for the size of the UE.
Furthermore, the use of the secondary antenna at the low frequency bands drains current and has an impact on the UE's complexity whilst providing insufficient gain. When a MIMO UE does an inter frequency cell change (e.g. cell reselection, handover procedure) from a high frequency band to a low frequency band it has to continue using MIMO. This in turn increases the complexity of the UE, performance loss and in some cases the UE behavior may be unclear.
MIMO is an advanced antenna technique to improve the spectral efficiency and thereby boost the overall system capacity. MIMO implies that both the base station and the UE employ multiple antennas. There exists a variety of MIMO techniques or modes such as Per Antenna Rate Control (PARC), selective PARC (S-PARC), transmit diversity, receiver diversity, Double Transmit Antenna Array (D-TxAA) etc. The D-TxAA is an advanced version of transmit diversity, which is already used in WCDMA for example as specified by 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”
The MIMO is used in all major technologies e.g. in WCDMA/HSPA, LTE, CDMA2000, Wi Fi/WLAN etc. For example in LTE there are 9 different MIMO techniques (aka transmission modes) specified. Most of these MIMO modes are mandatory for all UEs for all bands.
Irrespective of the MIMO technique the notation (M×N) is generally used to represent MIMO configuration in terms of the number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO and they correspond to transmit diversity and receiver diversity respectively. The configuration (2×2) will be used in WCDMA release 7.
In particular the WCDMA FDD release 7 will support double transmit antenna array (D-TxAA) in the downlink, which is a multiple input multiple output (MIMO) technique to enhanced capacity as disclosed by 3GPP TS 25.214, “Physical layer procedures (FDD)”.
The E-UTRAN downlink will indeed support several MIMO schemes including MIMO techniques including Single User-MIMO (SU-MIMO) and Multi User-MIMO (MU-MIMO) disclosed by 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”.
The MIMO technology has also been widely adopted in other wireless communication standards e.g., IEEE802.16.
The limited size of the UE results in limited distance between the antennas. As a result, the antennas may be optimized only for certain portion of the spectrum. The decorrelation between the antennas depends on the wavelength, in particular the distance between the antennas should be higher than a certain fraction of the wavelength, and the wavelength depends on the carrier frequency at which the UE is operating, L=C/f where C is the speed of light and the f is the frequency the wave is operating. For example, at f=2 GHz the wavelength is L=15 cm, while at 800 MHz L=37 cm and of course higher for lower bands. The optimal distance between the antennas is approximately equal to the wavelength of the carrier frequency divided by 2.
Therefore, due to the reduced size of the UEs, the required distance between the antennas and hence sufficient decorrelation cannot be achieved at low frequency, and hence all the benefits of multiple streams MIMO cannot be achieved. However the UE will continue to consume power if the secondary antenna is required to be active even in these low frequency bands.
When the UE supports MIMO, it signals this capability via RRC signaling to the network. This capability is valid for all the frequency bands the UE supports. The UE in general will support several frequency bands (high bands and low bands, depending on the deployment and on roaming). Hence the UE will have high power consumption for low gains when operating in the low frequency bands. Hence it is important to provide a method for the UE to change its capability when operating in a low frequency band.
Certain MIMO mode(s) may also be mandatory for all UEs. For example in LTE the MIMO transmission modes 1-8 (TM1-TM8) are mandatory for all UEs. This means UE does not signal its MIMO capability for these MIMO modes. However the introduction of large number of bands for LTE means that the UE has to support these MIMO modes for all supported bands. However this will be very challenging for the UE given the large differences between frequency ranges of the LTE bands. On the other hand TM9 is optional. Therefore UE signals its capability but the UE also has to support TM9 for all bands.
This is particularly true when a MIMO UE does an inter frequency cell change (e.g. handover procedure) from a high frequency band to a low frequency band in connected mode. Under these circumstances the UE cannot change autonomously its capability. This may cause multiple problems e.g. degradation of performance, loss of scheduling grant, UE implementation complexity since UE will have to implement MIMO on all bands etc.
Starting from rel-8 new UE capabilities have been introduced in 3GPP, mainly Multi-carrier HSDPA, where the UE is capable of receiving the signal on multiple carriers in the same time. Multi-carrier HSDPA can be also deployed with MIMO on each carrier to enhance further the data rate. In many densely populated areas such as hotspots an operator deploys more than one cell in the same geographical area, e.g. several cells in one sector. Each base station or Node B typically provides coverage to 3 sectors. As an example, a deployment with 2 carriers per Node B implies 2 co-located cells per sector and 6 cells per Node B.
In UTRAN system this corresponds to multiple cells 103 of 5 MHz each as shown in FIG. 1. Such cells 103 are also termed as ‘co-located cells’. The co-located cells 103 are served by the same base station or the Node B 101 over a plurality of carrier frequencies 105_1 to 105_n. 
Similar arrangement would be possible in E-UTRAN disclosed, for example, in 3GPP TS 36.101, “User Equipment (UE) radio transmission and reception (EDD)”. In E-UTRAN due to variable carrier frequency bandwidth the co-located cells may have different bandwidth and therefore they have different maximum transmission power levels. This is shown in FIG. 2. The base station or eNode B 201 serves co-located cells 203 over a plurality of carrier frequencies 205_1 to 205_n having difference bandwidths. However, even in E-UTRAN the co-located cells with the same bandwidth would still be the most common deployment.
In 3GPP currently several carrier aggregation configurations are defined both for UTRA and E-UTRA. Carrier aggregation can be defined for intra frequency deployments where the carriers can be located adjacent or non adjacent, or it can be defined for inter-frequency deployments where the aggregated carriers belong to different bands.
It can be noticed that the UE may need to support carrier aggregation where high and low bands participate in the configuration, such as for instance band I combined with band VIII. When the UE reports MIMO capability it is supposed to support MIMO on both the carriers. However, for a small sized UE scheduling MIMO won't bring the expected gains. Hence it would be beneficial for the UE to inform the network that MIMO is not supported for this particular band, to switch off the secondary antenna and to reduce battery consumption. The network can also reduce the extra complexity due to MIMO scheduling in this particular frequency band.
For downlink MIMO, after an inter-frequency handover, when the UE hands over a low frequency for which the size of the UE does not allow to have sufficient path decorrelation and when the UE is scheduled by considering multi band multi-carrier configuration where at least one low frequency participates in the carrier aggregation configuration. For uplink MIMO, after an inter-frequency cell change (e.g. handover), When the UE changes cell operating at a low frequency for which the form factor does not allow to have sufficient path decorrelation. When the UE is scheduled by considering multi band multi-carrier configuration where at least one low frequency participates in the carrier aggregation configuration and the uplink carrier is anchored to the low frequency band. This is applicable for both UTRA and E-UTRA.